1. Field of The Invention
The present invention relates to a demodulator for a multilevel amplitude modulation system, and more specifically to a demodulator for recovering a plurality of binary digital data from amplitude-modulated multilevel signals, and still more specifically to such a kind of demodulator which features effective compensation for undesired direct current drifts.
2. Description of the Prior Art
In order to increase the frequency spectrum efficiency in transmitting a digital signal by modulating a microwave carrier therewith, the multilevel (e.g. 16-level) QAM (quadrature amplitude modulation) system has proven to be effective.
In practice, however, the multilevel QAM system should be designed to meet strict requirements. One of the problems to be solved, is to compensate for direct current (DC) drifts of demodulated baseband signals which are applied to discriminators forming part of the demodulator.
A known approach to overcoming the DC-drifts of the demodulated baseband signals, has been disclosed in Japanese Patent Application No. 56-200047 (laid open under the publication No. 58-101449) in connection with a demodulator of the multilevel QAM system. According to this prior art, an error signal is derived from an analog-to-digital (A/D) converter which forms part of a discriminator and which is adapted to discriminate and recover the demodulated multilevel baseband signals. The error signal is fed back, via a DC voltage control loop, to a DC control circuit which precedes the A/D converter. The DC control circuit is responsive to the error signal and controls DC voltage components which are superimposed on the multilevel baseband signals, and thus compensates for the DC-drifts of the demodulated baseband signals.
On the other hand, in an effort to realize an improved demodulator of the multilevel QAM system, which features a simpler circuit configuration and low manufacturing cost, a demodulator (more precisely, a discriminator) utilizing a full-wave rectifier has been proposed. This full-wave rectifier is provided in front of an A/D converter and rectifies input signals with respect to zero voltage, so that the number of discriminating values is reduced by half and hence the the A/D converter is simplified or the number of the overall A/D converters involved in the discriminator can be reduced. This type of discriminator, however, is subject to DC-drifts within itself, viz., in an amplifier following the full-wave rectifier. These internally produced DC-drifts cannot be compensated for by the use of the control system as previously mentioned in connection with Japanese Patent Application No. 56-200047, in that this control system eliminates only the DC-drifts of the incoming baseband signals. In other words, desired circuit operation cannot be achieved only by the application of the known control system to the full-wave rectifier type demodulator or discriminator.
Hereinlater, the DC-drift superimposed on the baseband signal will be referred to as a first DC-drift, and that produced within the discriminator will be referred to as a second DC-drift.